This allows the joystick hotplug to function without the main event loop
(specifically: without SDL_INIT_VIDEO), and moves explicit polling for
joysticks where it belongs at the low-level: in SDL_SYS_JoystickDetect().
This lets apps call SDL_JoystickUpdate() to get hotplug events and keep
SDL_NumJoysticks() correct, as expected. As SDL_PumpEvents() (and
SDL_PollEvents, etc) calls SDL_JoystickUpdate(), existing apps will function
as before.
Thanks to "raskie" on the forums for pointing this out!
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